Transition Elements

Transition elements are the d-block metals occupying Groups 3-12 of the periodic table. The PMDC MDCAT 2026 syllabus asks for one thing here: the electronic structures of the d-block elements and their ions — including the Cr and Cu anomalies and the rule that 4s electrons are lost before 3d. This chapter is small but high-yield — expect 1-2 MCQs per paper.

PMC Table of Specifications. This chapter has one PMDC subtopic — Electronic Structure of d-block Elements (learning outcome 12.1: describe the electronic structures of the elements and ions of d-block elements). That single outcome is what we cover below.

Electronic Structure of d-block Elements

A transition element is defined as one whose atom or one of its common ions has a partially filled d-subshell. By this strict IUPAC definition, Zn (Group 12) is sometimes excluded because its 3d shell is full in both the atom and Zn²⁺. For MDCAT purposes, however, the entire d-block from Sc to Zn is studied as transition elements.

The 3d series (first transition series)

The first transition series spans atomic numbers 21-30 (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn). Their general electronic configuration is [Ar] 3d^1-10 4s^1-2. The 4s orbital fills before 3d (Aufbau), but 4s is the higher-energy orbital once 3d is occupied — so when these elements ionise, electrons are lost from 4s first, not 3d.

Configuration cheat sheet (3d series)

Sc: [Ar] 3d¹ 4s²
Ti: [Ar] 3d² 4s²
V: [Ar] 3d³ 4s²
Cr: [Ar] 3d⁵ 4s¹ — not 3d⁴4s²; half-filled 3d is extra stable
Mn: [Ar] 3d⁵ 4s²
Fe: [Ar] 3d⁶ 4s²
Co: [Ar] 3d⁷ 4s²
Ni: [Ar] 3d⁸ 4s²
Cu: [Ar] 3d¹⁰ 4s¹ — fully-filled 3d is extra stable
Zn: [Ar] 3d¹⁰ 4s²

Common trap. When forming Fe²⁺, electrons are removed from 4s first, giving [Ar] 3d⁶. Fe³⁺ = [Ar] 3d⁵ (half-filled, extra stable — this is why Fe³⁺ is the more stable iron ion in aqueous oxidising conditions). Many candidates wrongly write 4s²3d⁴ for Fe²⁺.

Electronic structure of d-block ions

Because the 4s electrons are lost first, the configuration of a d-block ion is worked out by removing 4s electrons before any 3d electrons — never by simply deleting the last electrons written in the Aufbau order.

Fe²⁺: Fe is [Ar] 3d⁶ 4s²; remove both 4s electrons → [Ar] 3d⁶
Fe³⁺: Remove 4s² then one 3d electron → [Ar] 3d⁵ (half-filled)
Cu⁺: Cu is [Ar] 3d¹⁰ 4s¹; remove the 4s electron → [Ar] 3d¹⁰ (fully filled)
Cu²⁺: [Ar] 3d⁹
Sc³⁺: [Ar] — an empty 3d subshell
Zn²⁺: [Ar] 3d¹⁰ — the 3d subshell stays full

Exam-favourite trap. "Why is Zn not a typical transition element?" The PMDC-friendly answer: its 3d subshell is completely filled (3d¹⁰) in both the neutral atom and its only common ion Zn²⁺, so it never has the partially filled d-subshell the definition requires.

Worked MCQs

Three MCQs covering the high-yield testing patterns for the electronic structure of d-block elements and their ions.

Q1. The ground-state electronic configuration of chromium (Z = 24) is:

[Ar] 3d⁴ 4s²
[Ar] 3d⁵ 4s¹
[Ar] 3d⁶
[Ar] 3d³ 4s² 4p¹

A half-filled 3d subshell is extra stable, so chromium promotes one 4s electron to give [Ar] 3d⁵ 4s¹ rather than the Aufbau-predicted 3d⁴ 4s². Copper (Z = 29) shows the same anomaly with [Ar] 3d¹⁰ 4s¹.

Q2. The number of unpaired electrons in Fe³⁺ is:

Fe is [Ar] 3d⁶ 4s². Fe³⁺ loses 4s² and one 3d electron, giving [Ar] 3d⁵ — five unpaired electrons by Hund's rule, and the half-filled 3d subshell contributes to the stability of Fe³⁺.

Q3. The electronic configuration of Cu⁺ is:

[Ar] 3d⁹ 4s¹
[Ar] 3d¹⁰
[Ar] 3d⁹
[Ar] 3d⁸ 4s²

Copper is [Ar] 3d¹⁰ 4s¹ in the ground state. Ionisation removes the 4s electron first, leaving [Ar] 3d¹⁰. Removing a 3d electron instead (giving 3d⁹ 4s¹) is the standard distractor; Cu²⁺ is the one that is [Ar] 3d⁹.

Quick Recap

Transition elements = d-block, partially filled d-subshell in atom or common ion.
Cr and Cu have anomalous configurations (3d⁵4s¹ and 3d¹⁰4s¹) due to half- and fully-filled stability.
Ionisation removes 4s electrons before 3d.
Fe²⁺ = [Ar] 3d⁶, Fe³⁺ = [Ar] 3d⁵, Cu⁺ = [Ar] 3d¹⁰, Cu²⁺ = [Ar] 3d⁹.
Zn is atypical: 3d¹⁰ in both the atom and Zn²⁺, so never a partially filled d-subshell.

Test yourself. Take a timed practice test or browse topic-wise Chemistry MCQs to lock these concepts in.